1. Field of the Invention
The invention relates to electronic circuits, and more particularly, to symmetrical polyphase networks.
2. Description of the Prior Art
As is well known to a person of ordinary skill in the art, symmetrical polyphase networks (PPN), or in a filtering context, polyphase networks (PPFs), have multiple usages in electronic circuit design. Generally speaking, symmetrical PPNs perform the same function as two separate resistor/capacitor (RC) networks, but are much less sensitive to component tolerances. In U.S. Pat. No. 3,559,042, which is incorporated herein by reference, Gingell discloses a symmetrical polyphase network (PPN).
FIG. 1 shows a conventional layout diagram of a 3-stage symmetrical PPN 100 according to Gingell's disclosure. The PPN 100 includes three stages 102, 104, 106 having resistors and capacitors coupled together in the same manner. As can been seen from FIG. 1, when viewed strictly from an electrical perspective, the PPN 100 has a symmetrical structure. In each stage 102, 104, 106, each node of the PPN 100 has connections to both a resistor and a capacitor. However, when viewed in conjunction with a physical layout structure perspective, such PPN is not symmetrical. As frequency increases, these connections between the resistors and capacitors have a greater and greater influence on the symmetrical nature of the design. More specifically, as frequency increases, the structure shown in FIG. 1 becomes less and less symmetrical. This is due to the fact that the connections themselves begin to influence the design. The symmetrical PPN 100 is typically implemented on a multi-layer substrate and vias are used in connection 108 to traverse the different layers. For example, in the first stage 102, in order to allow a connection 108 from capacitor CA4 to cross over multiple connections and couple to RA1, a minimum of two vias are used in connection 108 to traverse to a different layer, cross the multiple connections, and return to the original layer to couple to RA1. Hence, the length and number of vias in connection 108 are different than the length and number of vias in the other connections. At high frequencies, both the vias and the connections themselves have relatively significant resistance and capacitance properties. Therefore, at high frequencies, the operational efficiency of the PPN 100 becomes more dependent on the component values and is negatively influenced by the component layout and connection paths. A conventional solution to these problems is to use multiple stages, such as the three stages 102, 104, 106 shown in FIG. 1. By slightly varying the component values between each of the stages to overlap the frequency response of each stage at a selected frequency, the circuit designer can ensure the PPN 100 will correctly operate at the selected frequency. However, using multiple stages increases the component count, the cost, and the complexity of the design of the PPN 100.